Bias current generator circuits are well known in the art. Such circuits supply current for different sub-circuits of an integrated circuit.
An example of a prior art proportional to absolute temperature (PTAT) bias current generator 100 implemented using bandgap techniques is illustrated in FIG. 1a. The bias current generator 100 includes a first bipolar transistor Q1 operating at a first collector current density and a second bipolar transistor Q2 operating at a second collector current density which is less than that of the first collector current density. The emitter of the first bipolar transistor Q1 is coupled to the inverting input of an operational amplifier A and the emitter of the second bipolar transistor Q2 is coupled via a resistor r1 to the non-inverting input of the amplifier A. The output of the amplifier A drives a current mirror arrangement comprising two PMOS transistors of similar aspect ratios, namely, MP1 and MP2 which are biased so that their gate-source voltage Vgs are the same. MP1 and MP2 force equal currents to the emitters of the two bipolar transistors, Q1 and Q2. The collector current density difference between Q1 and Q2 may be established by having the emitter area of the second bipolar transistor Q2 larger than the emitter area of the first bipolar transistor Q1. Alternatively multiple transistors may be provided in each leg, with the sum of the collector currents of each of the transistors in a first leg being greater than that in a second leg. As a consequence of the differences in collector current densities between the bipolar transistors Q1 and Q2 a base-emitter voltage difference (ΔVbe) is developed across the resistor r1.
                              Δ          ⁢                                          ⁢                      V            be                          =                              kT            q                    ⁢                      ln            ⁡                          (              n              )                                                          (        1        )            Where:                k is the Boltzmann constant,        q is the charge on the electron,        T is the operating temperature in Kelvin,        n is the collector current density ratio of the two bipolar transistors.        
This base emitter voltage difference (ΔVbe) is inherently PTAT. Assuming that the amplifier A is an ideal amplifier, the emitter currents of Q1 and Q2 are given by equation 2.
                              I          ⁡                      (                                          Q                ⁢                                                                  ⁢                1                            ,              e                        )                          =                              I            ⁡                          (                                                Q                  ⁢                                                                          ⁢                  2                                ,                e                            )                                =                                    Δ              ⁢                                                          ⁢                              V                be                                                    r              ⁢                                                          ⁢              1                                                          (        2        )            
The bias current generated may then be used to bias the sub-circuits of an integrated circuit by typically mirroring the current which flows through r1.
                    Ibias        =                              Δ            ⁢                                                  ⁢                          V              be                                            r            ⁢                                                  ⁢            1                                              (        3        )            
Referring now to FIG. 1b, another prior art current generator, in this case, a complimentary to absolute temperature (CTAT) current generator 110 is illustrated. The bias current generator 110 is substantially similar to the bias current generator 100, and like components are identified by the same reference labels. The main difference between the bias current generator 100 and the current generator 110 is that a single bipolar transistor is coupled to the inputs of the amplifier A. The amplifier A forces the voltages at the non-inverting and inverting inputs of the amplifier A to be the same. The voltage at the non-inverting input is equal the base emitter voltage of Q1. Thus, the voltage at the inverting input is also equal to the base emitter voltage of Q1, which is inherently CTAT. Therefore, a CTAT voltage is developed across r1:
                    Ibias        =                              V            be                                r            ⁢                                                  ⁢            1                                              (        4        )            
The temperature coefficients (TC) of the PTAT and CTAT bias currents according to FIGS. 1a and 1b are influenced by the temperature dependence of resistors. Thus, the bias current is dependent on the value of the resistor r1. It will be appreciated by those skilled in the art that the resistance of resistors may vary from lot to lot of the order of +/−20%. As a consequence, the value of the bias current generated will also vary. A further disadvantage of the prior art current bias generators 100 and 110 is they occupy a large silicon area in an integrated chip. The resistor r1 is one of the primary reasons why the current bias generators 100, 110 occupy a large silicon area. A circuit which occupies a large silicon area is undesirable for low current applications.
Another drawback of resistor based PTAT or CTAT current generators is related to the trimming methods. In order to reduce output current variation due to process variation different methods are used such that the resistor value of r1 is trimmed for the desired output current. Laser trimming methods are used such that a small part of a resistor r1 is “polished” until the desired output current is achieved. Laser trimming is also used to blow a short metal link across a resistor, part of r1, such that the total resistance increases and the bias current decreases. The trimming part of the circuits adopted for laser trimming usually requires large die area. MOS transistors configured as switches are typically coupled in series or parallel with the resistor r1 such that the value of r1 can be digitally controlled. MOS transistors used as switches add errors and nonlinearity on the resulting bias current generated due to the finite value of their drain-source resistance and corresponding nonlinearity.
There is therefore a need to provide a bias current generator which provides a bias current without incorporating a resistor.